It has been said that, in a thermal power station and the like, carbon dioxide (CO2) is generated by combustion of fossil fuels such as coal to increase the concentration of CO2 in the atmosphere, leading to a rise in atmospheric temperature, and thus causing various environmental problems. In order to prevent global warming, the Kyoto Protocol was adopted at the Kyoto Conference to stop global warming (COP3) on December 1997. The Kyoto Protocol came into effect on February 2005, and measures for the reduction of CO2 emission have been implemented in each country. Among methods for recovering CO2 from a combustion exhaust gas containing oxygen (O2) and sulfur oxide (SOX) of the thermal power station and the like, a CO2 absorption method using an alkanolamine solution is exemplified as a method that would be put into practice in the near future. A study on the method has intensively been made from the 1990s, and types of alkanolamines suited for the recovery of CO2 and conditions of application to an exhaust gas containing sulfur oxide have been studied (Patent Document 1). When compared with monoethanolamines that have hitherto been used, these amines are less likely to be oxidized with oxygen contained in a combustion exhaust gas, however, in view of the use for a long time such as several thousands hours or more, the replenishment amount of a solution of these amines due to oxidative degradation of the solution would cause a problem. To cope with this problem, it has become possible to remarkably inhibit oxidation of amines in an absorbent by adding an oxidation-inhibiting substance (for example, an organic sulfur compound) of amines to a CO2 absorbent (Patent Document 2).
With respect to a device for the recovery of CO2 contained in a combustion exhaust gas of a boiler, research and development have been made, most widely, on a carbon dioxide recovery device using an absorber column that brings an aqueous alkanolamine solution into contact with an exhaust gas, and a desorber column that recovers CO2 by heating the amine solution that has absorbed CO2 (for example, Non-Patent Document 1).
When an aqueous solution of an alkanolamine is used as a carbon dioxide (CO2) absorbent recovery device, the alkanolamine in the absorbent is partially oxidized and decomposed by an oxidizing component such as O2 or NOx in a combustion exhaust gas, particularly oxygen to produce ammonia. It is known that ammonia is decomposed and produced from an alkanolamine, and also, there is a patent in which ammonia produced in the recovery of CO2 by the alkanolamine is used for denitration of a combustion exhaust gas (Patent Document 3). It is also known that alkylamins such as methylamine, ethylamine, and isopropylamine are respectively produced from alkanolamines such as 2-(methylamino)ethanol (MAE), 2-(ethylamino)ethanol (EAE), and 2-(isopropylamino)ethanol (IPAE), in proportion to the degree of oxidation of the alkanolamine, and thus the release amounts of these products are decreased by an oxidation inhibitor (for example, Patent Document 2). Patent document 2 discloses that water (H2O), oxalic acid ((COOH)2), and ammonia (NH3) are produced by an oxidation reaction of H2NCH2CH2OH (MEA) as shown in the following formula (1).H2C2H4OH+2O2→NH3+(COOH)2+H2O  (1)